The present invention relates generally to films and other articles that incorporate information whose appearance is highly dependent upon viewing angle.
Films that incorporate directional imagesxe2x80x94images that are viewable at some viewing geometries and not othersxe2x80x94are generally known. U.S. Pat. 6,024,455 (Oxe2x80x2 Neill et al.), for example, discloses reflective articles in which a multilayer film covers a patterned retroreflective layer. The patterned retroreflective layer can include an indicia layer having patterned regions comprising conventional inks, dyes, or other substances which are substantially opaque to some wavelengths but transparent to others. Such films, however, require specialized lighting arrangements for optimal viewing.
PCT Publication WO 99/36258 (Weber et al.) discloses, among other things, color shifting films with printed indicia, and optical brighteners such as dyes that absorb in the UV and fluoresce in the visible region of the color spectrum. Such articles can also provide images whose appearance changes with viewing geometry, particularly where the printed indicia is provided on a back side of the color shifting film with respect to an observer. Advantageously, such articles can be viewed under ordinary diffuse lighting conditions, such as in a typical office environment.
Close inspection of articles incorporating color shifting film and fluorescent indicia as described above has revealed a difficulty in satisfactorily concealing the fluorescent indicia from view. Articles having improved concealment of the fluorescent indicia are disclosed herein.
In disclosed embodiments, the article includes a color shifting film and indicia disposed behind the color shifting film from the point of view of an ordinary observer of the article. The indicia include a first and second colored portion arranged as a foreground and a background of the indicia. The first colored portion comprises a fluorescent colorant, and the second colored portion is substantially non-fluorescent. The second colored portion is selected to have substantially the same color as the first colored portion to enhance concealment of the indicia. Preferably, the first and second colored portions have the same color when viewed by themselves apart from the color shifting film, and also have the same apparent color when viewed through the color shifting film at an angle at which the color shifting film substantially blocks transmission of an emission band of the fluorescent colorant. In this way, the contrast between foreground and background is substantially reduced, and the indicia become less noticeable at such a viewing angle.